Open water risers are used to install, retrieve and work on conventional subsea Christmas trees. At the bottom of such risers can be found an emergency disconnect package (EDP) which includes a connector for emergency release of the riser from the Christmas tree and optionally, retainer valves on the riser bores to automatically close and retain fluid in the riser.
Conventional Christmas trees frequently are configured to have dual bores for the production tubing and for tubing annulus access. With the advent of deeper water systems, this has led to the use of monobore riser systems which require a bore selection device for pressure, wireline and coiled tubing access to either Christmas tree bore.
There are advantages associated with having a simple bore selection device which is located on the EDP. The operating mechanism of this device should ideally be reliable, field proven and flexible to the customer""s operating requirements, i.e. manual, hydraulic, fail safe to production, fail safe to annulus, providing position indication or automatic operation linked to the retainer valve functions.
The present invention provides a monobore riser bore selector comprising a sealed housing in which an unsealed guide is mounted for pivotal movement into selective alignment with each of a plurality of bores; a linearly movable stem being connected to the guide to cause said pivotal movement, the stem extending through a seal in the housing so that an end of the stem is positioned externally of the sealed housing, the externally positioned end being provided with a grab formation or being connected to an actuator stem extension for movement of the stem and the guide. For example, the bore selector may be moved into alignment with either a production bore or an annulus bore of a completion, as desired. The stem may be a standard ROV/manual operated gate valve operating mechanism. The ROV operation could be via torsion or linear actuation. In addition, standard gate valve UV stem and bonnet gasket sealing technology (as available from the applicants) can be used to isolate the bore selector cavity from the environment. This arrangement provides a reliable, flexible and field proven design.
Additionally or alternatively, the stem may be hydraulically actuated. The stem may be biased as desired to provide fail safe operation of the bore selector to the production bore position, to the annulus position, or to any other position.
Additionally or alternatively, the hydraulic circuitry of the stem actuator may be linked to that of the retainer valves, so that positioning of the bore selector is linked to the retainer valve functions (for example automatically pointing the bore selector at whichever valve is open if only one is open, or allowing bore selection by the operator if both retainer valves are open, or pointing at the last valve to open if both are open).
A further alternative is to mechanically link the bore selector for actuation by one of the retainer valve actuators, i.e. the stem actuator is one of the retainer valve actuators. For example, if the bore selector is mechanically linked to the production retainer valve, opening that valve could be arranged to cause the bore selector to point to the production bore. Conversely, closing the production retainer valve would then cause the mechanical linkage to align the bore selector with the annulus bore.
The bore selector is preferably a simple non-sealing tube or guide. It may be hinged for rotation at the top of the selector. At the bottom of the selector a suitable linkage such as a pivoting yoke or saddle may be used to transmit the horizontal stroke of the stem into the swinging motion of the selector. The linkage must permit vertical height change as well as horizontal stroke.
Further preferred features and advantages of the invention are set out below in the following description of illustrative embodiments, made with reference to the drawings.